1. Field of the Invention
The present invention relates generally to medical devices and methods for their fabrication. In particular, the invention relates to a transducer which has an improved external connection configuration for making an electrical connection to an imaging catheter.
2. Description of the Relevant Art
Intravascular imaging catheters, generally include one or more ultrasonic transducers which are capable of generating a high-frequency electrical signal, on the order of 30 MHZ, which is used to generate an image. The transducers may be front looking i.e. axially mounted so that an ultrasonic pulse is transmitted principally along the catheter axis, or side-looking i.e. mounted so that an ultrasonic pulse is transmitted in a direction perpendicular to the catheter axis.
In one example, a rotatable, side-looking ultrasonic transducer assembly 10, as shown in FIGS. 1A-1C, includes a single transducer element 12. The function of transducer element 12 is to produce and receive an ultrasonic beam. The beam is projected radially outward, normal to the surface of transducer element 12. Transducer element 12 is then rotated about an axis of the vessel to scan the interior of the blood vessel wall. Transducer element 12 detects reflections of the beam from the vessel wall which are converted to a cross-sectional image of the vessel.
Transducer element 12 is typically configured as a thin, rectangular sheet, which is fabricated from a piezoelectric material. Front and back surfaces of transducer element 12 are covered with thin film electrodes 14 and 16, respectively. As is well known, when a voltage is applied to electrodes 14 and 16, transducer element 12 vibrates to generate pulses at a resonant frequency determined by the mechanical and piezoelectric properties of transducer assembly 10. Conversely, when transducer assembly 10 receives an ultrasonic pulse, an imaging signal, in the form of a voltage pulse, is generated on electrodes 14 and 16 which may be amplified and transmitted to a video or other image generating system.
The external connection of leads to transducer assembly 10 to a lead 60, as shown in FIG. 1C, is of particular interest to the present invention. Lead 60 is coupled to the upper electrode 14 using a manually applied, conductive adhesive 18, usually an epoxy bond formulation including silver. The conductive epoxy 18 provides both a bond and an electrical path from the lead to upper electrode 14. In most cases, a matching layer 15, is formed over the upper electrode 14 prior to connecting external lead 60. In such cases, matching layer 15 must be formed from an electrically conducting material in order to provide the necessary conductive path between lead 60 and upper electrode 14.
Referring again to FIG. 1B, transducer element 12 is mounted within a receptacle 68 formed in a rotatable housing 24 using a bed of conductive adhesive filler 26. A second lead 62 is coupled to housing 24 which is electrically grounded to electrode 16 through the conductive adhesive filler 26. In this way, electrical connections to both electrodes 14 and 16 can be brought out through leads 60 and 62, respectively.
To form high-quality intravascular images, the active surface of the upper electrode 14, with or without matching layer 15 in place, should be kept relatively free of obstructions. The above-described external connection configuration has been successfully implemented in transducer assemblies that are used in relatively large diameter catheters. The presence of epoxy bond material 18 over the active surface of transducer element 12 has not typically interfered with image quality since the surface area occupied by bond material 18 is small relative to the remaining surface area of active transducer element 12 available for transmission and reception. Accordingly, preciseness and consistency in sizing and locating of the epoxy bond on the transducer element surface during assembly has been of minor importance.
To access small coronary and other arteries, however, "low profile" catheters must be used. These catheters have small diameters, which require the use of smaller transducer assemblies which have correspondingly smaller active surface areas. As the transducer element size is reduced, the relative space available on the active surface for external connection is substantially diminished. As the bond material occupies a proportionately larger amount of the available active surface area, aperture and resolution can be lost, thereby degrading the image and its usefulness. This, in turn, increases the importance of making smaller, uniformly sized, and precisely located bonds.
Unfortunately, reducing the bonding area used for external connection reduces the bond strength between the lead and the transducer and renders bond characteristics, such as electrical resistance, more variable. While improving the quality of the bond would help the problem, the ability of technicians to make smaller, consistently sized and precisely located bonds, without sacrificing the strength and quality of the connection is limited.
Current methods for external lead connection have still other disadvantages when used with very small transducers. For example, inconsistent silver epoxy bond and matching layer ingredient formulations cause variation in transducer performance. Furthermore, state-of-the-art bonding techniques cannot be used with the above described connection configuration. For example, wire bonding, soldering, and welding techniques cannot be used to attach leads to the silver epoxy bond, matching layer, or conductive film layers. Moreover, inconsistent size and placement of the bond causes significant part-to-part variation in performance and is not suitable for producing imaging catheters with repeatable characteristics.
For these reasons it would be desirable to provide improved transducer assemblies, methods for manufacturing such assemblies, and catheter systems incorporating the assemblies, where the transducer has an improved external connection configuration which limits obstruction of the active surface of the transducer assembly. It would also be desirable to provide the such assemblies and methods having the consistent bond resistance and other improved electrical characteristics and to remove the need for using an electrically conductive epoxy bond and/or a conductive matching layer. Finally, it would also be desirable to provide methods and assemblies with consistent part-to-part transducer element performance and allow for state-of-the-art external connection bonding techniques.